In response to Jeff's (re)discovery of low DARM cavity pole (alog 18283), I looked at past data of various DC and RF sideband power signals to see if there was something anomalous. Specifically speaking, I made a comparison between the data from May-1-2015 and the one from May-6-2015.

A first impression I had is that, looking at the AS 90 in-phase signals from the two data, the DRMI alignment looked very good to me and therefore I imagined that losses due to misalignment in SRC have been small. Since the DRMI for the 45 MHz sidebands is (almost) critically coupled, I can do an order estimation for SRC intra cavity losses. First, I simplifed the DRMI transmissivity as

  (amplitude transmissivity of DRMI to AS) = Ts / (1 + Ts + L) ~ 1 - L/Ts

where Ts and L are SRM power transmissivity and intracavity loss (or half of the round trip loss) respectively. AS 90 is made of beatnote of the upper and lower 45 MHz sidebands. So the AS90 should be proportional to the square of the above equation. In order to explain a 2 % reduction in the AS90 as shown in the above table, L needs to be about 350 ppm. This sounds too small in order for loss to reduce the DARM cavity pole as low as 270Hz (see the plot in alog 17889).

Perhaps, I should do a similar analysis for POP90 which showed some increase -- this is usually an indication of misalignment in SRC. However, on the other hand, POP18 behaved anomalously in the sense that it dropped by 40 % while the carrier recycling gain increased slightly. This shounds to me some rf saturation, beam clipping or that sort of things and therefore I am not so keen to analyze the POP signals of the 11 W data.